SynchronousQueue in ThreadPoolExecutor

Question

我正在尝试理解ThreadPoolExecutor中队列的行为。在下面的程序中，当我使用LinkedBlockingQueue时，每次只能向线程池提交一个任务。但是，如果我用LinkedBlockingQueue替换SynchronousQueue，我可以立即将所有5个任务提交到池中。在这种情况下，SynchronousQueue与LinkedBlockingQueue有什么不同？

Java程序:

import java.util.concurrent.Executors;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.SynchronousQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Sample {
    public static void main(String[] args) throws InterruptedException {
        LinkedBlockingQueue<Runnable> threadPoolQueue = new LinkedBlockingQueue<>();
//      SynchronousQueue<Runnable> threadPoolQueue = new SynchronousQueue<>();
        ThreadFactory threadFactory = Executors.defaultThreadFactory();
        ThreadPoolExecutor tpe = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, threadPoolQueue, threadFactory);
        Runnable np;

        for (int i = 1; i <= 5; i++) {
            np = new SampleWorker("ThreadPoolWorker " + i);
            tpe.submit(np);
        }

        System.out.println(tpe.getCorePoolSize());
        System.out.println(tpe.getPoolSize());
        System.out.println(tpe.getActiveCount());

        tpe.awaitTermination(Long.MAX_VALUE, TimeUnit.MILLISECONDS);
        tpe.shutdown();
        System.out.println("Main task finished");
    }
}

class SampleWorker implements Runnable {
    private String workerName;

    SampleWorker(String tName) {
        workerName = tName;
    }

    @Override
    public void run() {
        try {
            for (int i = 1; i <= 10; i++) {
                Thread.sleep(3000);
                System.out.println(this.workerName);
            }
            System.out.println(this.workerName + " finished");
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

Answer 1

当您向ThreadPoolExecutor提交任务时，它的工作方式如下：

if (numberOfWorkingThreads < corePoolSize) {
   startNewThreadAndRunTask();
} else if (workQueue.offer(task)) {
   if (numberOfWorkingThreads == 0) {
       startNewThreadAndRunTask();
   }
} else if (numberOfWorkingThreads < maxPoolSize)
    startNewThreadAndRunTask();
} else {
    rejectTask();
}

• 当使用没有初始值的LinkedBlockingQueue时，workQueue.offer(task)总是成功的，只会启动一个线程。
• 当调用SynchronousQueue.offer(task)时，它只有在另一个线程等待接收它时才会成功。因为没有等待线程，所以每次都会返回false并创建新线程。